Depth perception, or the capacity to perceive the environment in three dimensions, is based on the fact that the right and the left eye of a viewer look at the environment from a slightly different point and from a slightly different angle. Therefore, the images perceived by the right and the left eye differ from each other to some extent, and the difference between said images makes it possible to perceive the surrounding space in three dimensions.
It is known from prior art to record images in such a way that the 3-dimensional impression corresponding to the image recording situation is transmitted to the viewing person. These techniques are generally referred to as stereo imaging, and they are thus, in one way or another, based on the fact that a slightly different image is transmitted to the viewer's right and left eye.
One well-known example of the use of stereo imaging is Viewmaster™ viewing devices which make it possible to look at still images recorded on a photographic film separately for the right and the left eye by means of a viewing device to be placed in front of the viewer's eyes. The stereo images are recorded on the film by means of a special camera using two separate objective lenses.
Stereo imaging is used not only for presenting still images but also moving images.
From prior art, solutions are known for projecting a stereo image onto a screen by means of a film projector, wherein the same image can be looked at by several viewers, for example in a cinema. In this case, the viewers wear special eyeglasses to prevent the image intended for the right eye from being visible to the left eye, and vice versa. In the case of film projectors, the function of said glasses may be based on either polarization or colour filtering; in other words, the images intended for the right and the left eyes are encloded differently by means of polarization or colour coding. The image to be projected on the screen simultaneously includes superimposed images which have been encoded differently for the right and the left eye and from which, thanks to the special eyeglasses worn by the viewer, different images are conveyed to the viewer's right and left eye, to produce a 3-dimensional impression.
At present, when image material is primarily in electronic format and can thus be reproduced, for example, by means of a video signal, video projectors are used instead of film projectors to project a stereo image onto a screen or another projection surface. The image material to be presented by video projectors may be a stereo image recorded by special cameras with two objective lenses, or an image edited from an ordinary image by means of image processing, or also a stereo image developed in a completely “artificial” manner by means of a computer. At present, the projection of stereo images is mostly used in the reproduction of image material produced by a computer, for example to create various virtual environments.
A stereo image can be transferred and reproduced in electronic format by means of a special stereo video signal, in which the images intended for the right and the left eye are arranged to alternate one after the other in time. Consequently, the image frequency of the stereo video signal is double compared to the image frequency of a conventional video signal, if the image frequency of the images intended for the right and the left eye is to be maintained at the normal image frequency level (25 or 30 images per second).
Consequently, when said stereo video image is presented to a viewer by means of a wide screen television projector, the images intended for the right and the left eye are thus projected in an alternating manner one after the other in time. To control these successive images to be perceived in the correct manner by the viewer's right and left eye, the viewer wears so-called shutter glasses, whose operation is synchronized for showing images in succession. The shutter of the shutter glasses in front of the right eye is shut when the image intended for the left eye is projected, and in a corresponding manner, the shutter in front of the left eye is shut when the image intended for the right eye is projected. The operation of the shutter glasses is typically based on the use of liquid crystal shutters.
When a single video projector is used for projecting a stereo video signal, this will set considerable demands on the video projector, because the video projector must now, in principle, be capable of operating at a double image frequency (50 or 60 images per second) compared to the normal image frequency (25 or 30 images per second). When a computer image is being displayed, the image frequency established for images intended for one eye is even higher than for normal video images, 60 images per second, wherein the image frequency for stereo images becomes as high as 120 images per second.
Because of this, single video projectors which are suitable for projecting a stereo video signal are thus based on the CRT (cathode ray tube) technology, well known as such, which makes such high image frequencies possible but which also makes the structure of projectors relatively complex and thereby expensive. For example, liquid crystal video projectors, which are less expensive and otherwise considerably simpler from the technical point of view, cannot be used for this purpose in a corresponding manner, because due to the lag of liquid crystal matrix elements they do not have the capacity for the sufficiently high projection rates required by the double image frequency.
For the above-mentioned reason, solutions have also been developed, in which image signals intended for the right and the left eye are separated from the stereo video signal to two different video projectors. Thus, instead of special CRT projectors, the video projectors used can be less expensive projectors operating at a normal image frequency, for example projectors based on the liquid crystal matrix. The images of said two projectors are focused on top of each other, superimposed simultaneously by crosswise light polarizations, and the image thus formed is viewed through eyeglasses polarized crosswise differently for the right and the left eye. Such an arrangement is known, for example, from the publication WO 00/55687.
However, such solutions of prior art, using two projectors, are limited expressly to the use of polarization (or colour filtering) to separate the images intended for the right and the left eye from each other. In other words, in these systems, the images intended for the right and the left eye are projected to the viewer simultaneously and not sequentially in time as in systems using shutter glasses.
However, the use of polarization or colour filtering involves obvious disadvantages when compared with methods based on shutter glasses. In systems based on polarization, for example, tilting of the head by the viewer will change the filtering capacity of the polarization glasses worn by him/her. This is problematic when a stereo image is used, for example, to create a virtual environment, in which virtual environment the viewer is expected to move in relation to the stereo image projected to him/her. Both in systems using polarization and in those using colour filtering, the natural reproduction of colours is naturally poorer than in systems based on shutter glasses, in which the colour reproduction is substantially neutral when the liquid crystal shutters of the shutter glasses are open. Also, both the polarization and the colour filters significantly reduce the brightness of the image perceived by the viewer, because the image is now typically both projected and viewed through optical filters with a limited transmission.
In systems based on shutter glasses and using two projectors, the problem in practice is that the operation of the projectors should be precisely synchronized with each other to achieve a high image quality. In other words, the projector showing the image intended for the right eye must, in principle, not project the image when the other projector is projecting the image intended for the left eye. In spite of liquid crystal shutter glasses, or the like, worn by the viewer, if the video projector producing the image intended for the left eye projects the image or otherwise emits light when the shutter of the shutter lense in front of the right eye is open, it can also be seen by the right eye, because in this case polarization or colour filtering is thus not used for the encoding of images. The above-mentioned unwanted “leaking” of the image or light crosswise to the other eye will naturally impair the depth impression and contrast of the stereo image as well as cause flicker of the image.
In liquid crystal video projectors, in which the light transmitted by a light source penetrates the liquid crystal matrix, and the image formed onto the liquid crystal matrix is projected by means of lense optics further to be superimposed on a projection surface, the limited rate of operation of the liquid crystal matrix will make the above-described synchronization of the images more difficult. Furthermore, a perfect contrast cannot be produced by the liquid crystal matrix, whose operation in the projector can be compared with the function of a slide in a slide projector. In other words, even though the aim is to control the liquid crystal matrix to be non-transparent, i.e. to reproduce a completely black image, a given portion of the light from the light source is always passed through the liquid crystal matrix and further onto the screen. This unwanted light impairs the quality of the stereo image produced.
Because of these facts, the production of a high-quality stereo image by means of two separate video projectors and the technique based on shutter glasses always requires, in practice, the use of video projectors which are of a relatively high quality and thereby relatively expensive. This is emphasized particularly in the reproduction of a stereo image produced by means of a computer, in which the total image frequency of the stereo image is even higher than that of a stereo image formed of a normal video image. Thus, in practice, the advantage to be achieved in the reproduction of a stereo image by means of two projectors in relation to the use of one special projector based on the CRT technology remains small, or, accordingly, the quality of the stereo image remains significantly poorer than the image quality achieved with the special projector.